The human and mouse MHC's are important for their intricate roles in regulating immune processes, but are also very complex genetically and structurally. We are in the process of better defining known HLA antigens, identifying new variants, and looking for new HLA loci which cannot be determined serologically but only through functional assays such as the Hutterites and Amish. The influence of the MHC on differentiation and the effects of regulatory genes on NHC expression and function are being studied in the mouse, where more detailed knowledge of H-2 and H-2 associated genes is available. Of particular interest are the genes between T and H-2.